The present invention relates to electrical power supply systems and, more particularly, to load shedding apparatus for battery storage power supplies.
Some power supply systems which include battery power storage may experience load demands which periodically exceed the energy capacity of the system. Such systems include spacecraft systems in which a battery storage system is charged by a solar array.
The problem becomes especially troublesome in spacecraft in orbits where substantial eclipse periods may cut off the solar array from insolation. If the load being supplied by the battery system becomes high during an eclipse period, the energy remaining in the battery system may be reduced to a value below that at which activities necessary to the survival of the spacecraft may be performed. For example, it is vitally necessary that the spacecraft maintain attitude control since loss of attitude control may permit the spacecraft to begin tumbling. This could be fatal since it may prevent the solar array from again acquiring sufficient energy to restore the battery charge during the ensuing period of access to solar energy. Thus the loss of function could be permanent.
In order to avoid potentially fatal drainage of spacecraft storage batteries, it has become conventional to divide the spacecraft electrical load into an interruptible load and an uninterruptible load. In a communications satellite, for example, attitude control and certain necessary housekeeping activities may be defined as the uninterruptible load which must remain supplied with the remaining power in the battery system regardless of the actual amount remaining. The communications task of the satellite may be defined as the interruptible load since, although lack of communications for a period may be inconvenient, the alternative of complete and permanent loss of spacecraft utility is generally considered too high a price to pay for a limited period of continued communications.
Ideally, the interruptible load is shed when the battery system contains an amount of charge remaining that is sufficient to enable the spacecraft system to survive until a positive battery charging condition is achieved. That is, load shedding should be performed when the battery system has reached a predetermined state of charge. This permits continuing the spacecraft mission (e.g. communications) for as long as possible without jeopardizing the survival of the system.
The state of charge of some battery systems can be determined or inferred from measurements. For example, the state of charge of a lead-acid battery can be determined from a measurement of the specific gravity of its electrolyte. Unfortunately, the state of charge of nickel-cadmium batteries, which are favored in spacecraft applications, cannot be explicitly determined from internal measurements or from terminal conditions. The terminal voltage, for example is a non-linear function of charge which is dependent on a complex function of temperature, battery age, battery usage history and battery state of charge. Particularly near the fully charged state, the amount of charge added to a nickel-cadmium battery per ampere hour fed to it decreases rapidly and non-linearly. Also, near the fully charged state, the terminal voltage rises rapidly to a maximum value.
Due to the uncertainty of battery state of charge, prior art load shedding systems depend principally on simple timing strategies to select a time for load shedding. This, of course, risks errors of two types. The most serious error is that of delaying load shedding beyond the time at which sufficient charge remains to sustain vital functions until a positive charging rate is again achieved. The less serious error is unnecessarily interrupting the spacecraft mission when more than sufficient charge remains to survive the anticipated charging deficiency period.